Modern oscillation dampers for motor vehicles are provided with electric control means for variation of the damping effect in accordance with the driving conditions. These control means may either be automatic control means which change the damping effect of the oscillation damper in accordance with measured driving parameters such as the amplitude of certain measured oscillation or, alternatively, the driver may control the damping effect according to his desire. For comfortable driving conditions a relatively small damping effect is usually desired, whereas for sportive driving conditions a stronger damping effect is usually requested. If the damping effect becomes too low the security of the vehicle and of the driver is reduced in case of critical driving situations such as rough road surfaces. As the electric control of the oscillation damper may fail it is necessary to make sure that in such case of failure of the electric control the damping effect is adjusted to a predetermined value. As after a failure of the electric control of the oscillation damper there may still occur critical situations such as driving on rough road surface it is necessary to adjust the behavior of the oscillation damper such that the selected adjustment is still in accordance with the safety requirements under such critical driving conditions. It is therefore usual to automatically adjust the damping behavior of an oscillation damper to a relatively strong damping effect in case of failure of the electric control of the oscillation damper. For a long time one has believed that it was necessary to automatically adjust the damping behavior in case of failure of the electric control to the strongest possible damping effect. It has been found, however, (see, for example German patent publication 39 17 064-A1) that it is not always necessary to select the strongest damping effect in case of failure of the electric control. It is rather possible to select an intermediate value of damping effect in case of failure of the electric control. In the present invention both possibilities are considered.
The adjustment of the damping behavior in case of failure of the electric control is as a rule effected by elastic biasing means which displace the components of a control unit which are responsible for adjustment of the damping behavior to a predetermined position when the electric control is interrupted or failure conditions of similar consequences occur. For selecting an adequate damping behavior under normal operational conditions it is necessary to maintain the above mentioned components in an operational position different from the basic position by electro-magnetic transferring means (nominated EM-transferring means in the following description). Such energy is consumed under normal driving conditions for maintaining these components in the respective operational position.
As under normal driving conditions frequently a soft damping effect is desired, for making the vehicle comfortable, the operational damping conditions are during the larger part of the total time of operation of most vehicles softer than the damping conditions to be used in case of failure of the electric control. This means that a considerable amount of energy is dissipated for maintaining the above said components in the operational position corresponding to comfortable driving conditions.